1. Field of the Invention
This invention relates to digital data recording, and more particularly, to a method and apparatus for recording compressed data on digital audio tape (DAT) media.
2. Description of Related Art
Magnetic tape storage devices are widely used for the storage of large amounts of digital data, because they provide an economical and reliable means for temporary and permanent storage. Because magnetic tape systems inherently rely on serial recording, access times are substantially longer than other storage devices, but at the same time the danger of catastrophic failure is virtually absent. Thus, it has become common practice to utilize tape systems as data backup for floppy disk and hard disk files.
Tape drive systems have evolved over the recent past with technical improvements that have resulted in substantial increases in capacity accompanied by significant decreases in size. Large self-contained tape transports using parallel track recording techniques and relatively wide tape are well known in the art, but these devices are incompatible in size, cost and power requirements with the compact computer systems that are now employed. Further, threading of tape in reel-to-reel devices has always been a cumbersome task, so that efficient tape cartridge systems have been invented for compact computer systems.
More recently, helical scan recording techniques originally devised for video recording have been adapted to provide high density, high fidelity, digital audio tape recordings. A standard digital audio tape (DAT) format has been adopted in order to achieve uniformity in the mass production and marketing of entertainment oriented materials in the audio portion of the frequency spectrum. In addition, a digital data storage (DDS) format for DAT media has been standardized as described, for example, in the publication: American National Standards, "Digital Data Storage (DDS) Format For Information Interchange", 8th Draft, Sep. 28, 1990, which publication is incorporated herein by reference.
Refer now to FIG. 1, which is a diagram describing the standard format and layout of control information and data on DAT media. A helical-scan tape deck passes a tape at a predetermined angle across a rotary head drum with a wrap angle of 90.degree.. The head drum typically houses two read/write heads which are angularly spaced by 180.degree.. The heads write overlapping oblique tracks 10 and 12 across the tape. The track 10 written by a first head has a positive azimuth, while the track 12 written by a second head has a negative azimuth. Each pair of positive and negative azimuth tracks 10 and 12 constitute one frame 14.
Each track 10 and 12 comprises two marginal areas 16, two subcode areas 18, two automatic track following (ATF) areas 20, and a main area 22. The subcode areas 18 are primarily used to store auxiliary information, for example, as to the logical organization of the main area 22, its mapping on to the tape, certain recording parameters (such as format identity, tape parameters, etc.), and tape usage history. The ATF areas 20 provide signals enabling servos to control the heads so that they accurately follow the tracks. The main area 22 is used primarily to store the data provided to the apparatus, although certain auxiliary information is also stored in this area.
Refer now to FIG. 2, which is a diagram describing the DDS format for DAT media, wherein each frame 14 illustrated therein consists of two tracks as indicated in FIG. 1. In the DDS format, data is stored in the main area 22 as a sequence of groups 24 on the tape, each group 24 having a fixed number, i.e., 22, of frames 14 and thus a fixed capacity. Each frame 14 stores 5756 bytes of data, so the capacity of a group 24 is 126,632 bytes of data. An index section 26 in each group 24 identifies the records, file marks, and save set marks contained in the group 24. Adding this level of indexing allows for variable length records and marks to be encoded on to the tape in the most efficient manner possible. Each group 24 may also be followed by additional error correction data comprising a frame 14 of ECC data.
The index section 26 is divided into two parts: the Block Access Table (BAT) 28 and the Physical Information Table (PIT) 30. The BAT 28 describes the contents of the group 24 and contains an entry 32 for each record, file mark, and save set mark. The size of the BAT 28 varies depending upon the contents of the group 24. The PIT 30 contains a list of counters and pointers describing the characteristics of the group 24, including the number of entries 32 in the BAT 28, the file mark count, the file mark count, the save set count, the record count in the group 24, etc.
Using the DDS format, each 60 meter cartridge has a data capacity of 1.3 gigabytes so that at a sustained transfer rate of 183 kilobytes/second there is a capacity for receiving 2 hours of data, equivalent to the contents of two large 650 megabyte disk drives. Thus, the DDS format for DAT media provides a significant enhancement and improvement to the art of digital recording.
Compression schemes have been devised so as to make even more efficient use of DAT media. Compression is typically applied to data prior to applying the DDS format so that compatibility with the DDS format is maintained. Thus, compression may be performed on each record, or, a number of records are combined into a compressed super-record, prior to arranging the records into frames 14 and groups 24.
Prior compression schemes, however, suffer from numerous disadvantages. For example, record compression is often inefficient because of the relatively small size of the records. Further, the BAT and PIT are not compressed. In addition, although unused space within a record may be compressed, unused space within a group 24 will not be compressed. Also, the size of the uncompressed data read from the DAT media cannot be known in advance and therefore memory management is complicated. Thus, whatever the merits of these prior compression schemes, they do not achieve the benefits of the present invention.